There are a variety of fuel sources for extracting energy for useful work, such as generation of steam, heat, and electricity, for example. Fuel sources generally are cost components that incur expenses in procurement, processing for use as a fuel, transport, storage, and use. Such fuels include coal, natural gas, and the like.
Alternative fuel sources include waste by-products from manufacturing and processing, for example, fibers from agricultural processing. Use of waste provides the economic benefit of extraction of energy from a material that otherwise may have disposal costs and no other practical benefit. Fiber is used herein to refer to any material derived from a member of the plant kingdom that has been physically separated or at least partially depleted (i.e., to less than about 40% d.s.b. total) of sugars, starch, protein, and germ. Separated fiber includes, but is not limited to, husks, hulls, nutshells, leaves, trunks or stalks, branches, roots, sprouts, chaff and dust. Fiber can be burned or combusted to provide energy; depending on the plant source, the fiber's composition, and its water content, it typically has a higher heating value (HHV) of about 7000-10,000 BTU/lb (dry basis). Other plant materials, such as corn germ, can have HHVs as high as 15,000 BTU/lb (dry basis). Therefore, theoretically, fiber or other plant material could be burned in order to wholly or partially power industrial processes, such as a milling process which produces fiber as a coproduct.
However, fiber has generally not been used as an energy source for a number of reasons. First, most fiber contains relatively high levels of ash (inorganic ions), such as elementary or compounds of phosphorous, calcium, magnesium, sodium, and potassium. For example, typical corn kernel fiber contains about 4% d.s.b. ash, of which phosphorous is the most common element (total ash containing about 40 wt % P2O5). Fiber ash generally has a relatively low fusion or melting point, meaning that at higher temperatures the ash is molten and will form slag on refractory or metallic surfaces of a furnace, boiler, or flue gas stack if the molten ash contacts these surfaces. Ash at temperatures below its melting point is generally in the form of small, generally irregularly shaped, solid particles.
Another concern regarding fiber combustion is fuel-bound nitrogen. Fiber typically contains some residual proteins, which contain nitrogen. Combustion in air or under another oxygenated atmosphere of a fuel containing nitrogen will lead to formation of nitrogen oxides (NOx). NOx emissions are generally discouraged under the regulatory climates prevailing in the developed world. NOx emissions can be partially reduced through good combustion practices. NOx can be further reduced from flue gas by various known means, but these involve further expenses for equipment, maintenance, and operations.
There are different systems for processing biomass materials and fibrous materials for extracting useful components. Generally the processes are either wet or dry. For example, in wet mill processing of corn, the corn is soaked in water and milled to form a mash. The process then separates the mash into fiber, starch, protein, corn germ, and byproduct fibrous materials. The process thereby separates the valuable and waste byproducts of the corn. The waste byproducts lack food or other commercial value, but have inherent BTU value. Dry separation of biomaterials is generally not as effective as wet processing in separating desirable components from waste products.
However, a drawback to wet mill processing is that the waste products are wet and therefore difficult to recover energy. By comparison, wood chips are usually readily combustible. However, other waste byproducts have decreasing commercial value and increasing difficulty in extracting energy. For example, rice hulls contain high quantities of silicate. Other materials are difficult to burn or contain hazardous material. The value of the waste product decreases as the difficulty to recover energy from the waste byproduct increases.
Slagging also increases the difficulty of recovering energy from waste byproducts. Slag is a built-up material formed as a byproduct of combustion. Slag results where the noncombustible material softens or melts into a molten sticky layer that builds up on the interior walls of the combustion unit. Slag is difficult to extract and remove.
Accordingly, there is a need in the art for an improved combustion system and apparatus for extracting energy from fiber that has minimal risk of slagging and relatively low NOx emissions. It is to such that the present invention is directed.